Method for Producing a Hydrogen-Enriched Gas Stream from Hydrogenated Gas Streams Comprising Hydrocarbons

ABSTRACT

The invention concerns a method for producing a hydrogen-enriched gas stream from at least one gas stream comprising for the major part hydrogen and a gas stream comprising hydrogen and hydrocarbons, using a pressure-modulated gas adsorption unit, the temperature of the gas stream comprising hydrogen and hydrocarbons being first lowered so as to condense the hydrocarbons.

The present invention relates to a process for enhancing various streamsbased on hydrogen and on hydrocarbons so as to produce a high-purityhydrogen stream.

Refineries or petrochemical plants use large amounts of hydrogenatedstreams, preferably of high purity. In many industrial cases, theperformances of the units, and especially the grades of the productsproduced, are limited by the purity of the supplied hydrogen.Furthermore, in these units, it is sometimes necessary to eliminategases that are still rich in hydrogen under the effect of the recyclegas purges. This results in a cost premium for the operation of theunit. In order to avoid these problems, processes for recovering thehydrogen from streams of relatively low purity in order to producehigh-purity hydrogenated streams have been proposed, especially inApplication WO 2005/042640 which proposes using a pressure swingadsorption unit to treat these various hydrogenated streams and toobtain a high-purity stream. This type of pressure swing adsorption unitmay represent a significant investment in terms of adsorbent cost andcompressor cost.

The objective of the present invention is to propose a process forenhancing various streams based on hydrogen and hydrocarbons so as toproduce a high-purity hydrogen stream, a liquefied petroleum gas and awaste gas, the investment cost of which is reduced by increasing thehigh-purity hydrogen production yield.

Another objective of the present invention is to propose a process forenhancing various streams based on hydrogen and hydrocarbons so as toproduce a high-purity hydrogen stream, a liquefied petroleum gas and awaste gas, of which the energy consumption is optimized.

For this purpose, the invention relates to a process for producing ahydrogen-enriched gas stream and a liquefied petroleum gas from at leastone gas stream mainly comprising hydrogen at pressure P and a gas streamcomprising hydrogen and hydrocarbons, in which the following steps arecarried out:

-   -   during step a), the gas stream comprising hydrogen and        hydrocarbons is adjusted to the pressure P;    -   during step b), the gas stream comprising hydrogen and        hydrocarbons and adjusted to the pressure P is treated in a        pressure swing gas adsorption unit (U) fed will the gas stream        mainly comprising hydrogen and having a pressure P so as to        supply:        -   at a first outlet, the hydrogen-enriched gas stream having a            hydrogen concentration greater than that of the gas stream            mainly comprising hydrogen; and        -   at a second outlet, a waste stream,            characterized in that between step a) and step b), the            temperature of the gas stream comprising hydrogen and            hydrocarbons and adjusted to the pressure P is lowered so as            to condense the hydrocarbons and form the liquid petroleum            gas.

The process according to the invention allows the treatment of two gasstreams comprising hydrogen at various concentrations so as to produce ahydrogen-enriched gas stream, a liquefied petroleum gas (or LPG) and awaste gas. The first gas stream treated is a gas stream that mainlycomprises hydrogen, that is to say having a hydrogen concentrationbetween 50 and 99 vol %. This stream also comprises hydrocarbons havingfrom 1 to 8 carbon atoms and compounds such as CO, CO₂, H₂S, etc. Thesecond stream treated is a gas stream comprising hydrogen andhydrocarbons having a hydrogen concentration at least 10% lower,preferably at least 15% lower and more preferably still 15 to 50% lower,relative to the value of the hydrogen concentration of the gas streammainly comprising hydrogen. This second stream also compriseshydrocarbons having 1 to 8 carbon atoms, and also compounds such as H₂O,H₂S, etc. Generally, this second gas stream has a hydrocarbonconcentration between 10 and 50 vol %. According to the invention, thepressure of this second gas stream comprising hydrogen and hydrocarbonsis adjusted so that it is close to 2, either by compression, or bypressure drop depending on the origin of this stream. It is alsopossible that this stream already has a pressure P, in this case it isused as is without a pressure adjustment. These various streams may bederived from the purge of the gas loop of a hydrotreatment unit such asa hydrodesulfurization or hydrocracking unit of a refinery, or else suchas a hydrodealkylation or isomerization unit of a petrochemical site.The first and second gas streams treated in the present invention may bederived from mixtures of various purges of such units. These mixturesmay make it possible to attain the pressure P desired by combining thestreams having a pressure below P with streams having a pressure aboveP.

By treatment of these various streams, the invention makes it possibleto enrich the gas stream mainly comprising hydrogen. This enrichment isobtained by hydrogen depletion of the second gas stream comprisinghydrogen and hydrocarbons. The unit thus produces the hydrogen-enrichedgas stream generally having a hydrogen purity greater than 99 vol %, andthe unit also produces a waste stream of low hydrogen purity and of lowpressure which may be conveyed to the fuel gas network. The pressure andhydrogen concentration of the waste stream are respectively below thepressure and hydrogen concentration values of all the effluents goinginto the unit (U). This waste stream also comprises hydrocarbons (C₁ toC₈) and compounds such as H₂O, H₂S, CO, CO₂, etc.

Preferably, the gas separation unit (U) is a pressure swing adsorption(PSA) unit combined with an integrated compressor, in which, for eachadsorber of the unit, a pressure swing cycle is carried out comprising asuccession of phases which define the phases of adsorption,decompression, purge and pressure rise, such that:

-   -   during the adsorption phase:        -   during a first step, the gas stream mainly comprising            hydrogen and having a pressure P is brought into contact            with the bed of the adsorber; and        -   during a second step, introduced into contact with the bed            of the adsorber is the mixture of pressure P whose            temperature has been lowered so as to condense the            hydrocarbons and that is composed of:            -   on the one hand, the gas stream comprising hydrogen and                hydrocarbons; and            -   on the other hand, the PSA recycle gas,        -    so as to adsorb the compounds other than hydrogen and to            produce, at the top of the adsorber bed, the enriched stream            having a hydrogen concentration greater than that of the gas            stream mainly comprising hydrogen;    -   during the decompression phase, the PSA waste stream is        produced;    -   during the purge phase, a purge gas is produced; and    -   where the PSA recycle gas is either the waste stream compressed        to the pressure P, or the purge gas compressed to the        pressure P. According to this PSA process, in a first adsorption        phase, the gas stream mainly comprising hydrogen is brought into        contact with a first PSA adsorbent bed and in a second phase, it        is the mixture of streams comprising hydrogen and hydrocarbons        and of the PSA recycle gas which are brought into contact with        this first adsorbent assembly. The recycle gas may be composed        of two gases, alone or as a mixture: the waste gas resulting        from the PSA which has been compressed, and the purge gas        resulting from the PSA which has been compressed. Preferably, it        is the purge gas and not the waste gas. The waste gas is derived        from the final step of the PSA decompression phase and is partly        compressed by the compressor integrated into the PSA unit of the        gas separation unit (U) whereas the purge gas is derived from        the purge phase of the PSA and is partly compressed by this same        compressor integrated into the PSA before being used as a        recycle gas. These two gases both comprise hydrogen and mainly        impurities. Once compressed, they are mixed with a stream        comprising hydrogen and hydrocarbons. This mixing may be carried        out in various ways depending on the pressure value of the        stream comprising hydrogen and hydrocarbons. This stream        comprising hydrogen and hydrocarbons may be mixed with the waste        gas or with the purge gas, then this mixture may be compressed        by the compressor integrated into the PSA unit to the        pressure P. When the stream comprising hydrogen and hydrocarbons        has a pressure above P, its compression may be avoided; in this        case, only the waste gas or the purge gas is compressed to form        the recycle gas. Before its introduction into the PSA unit, the        mixture at pressure P of the stream comprising hydrogen and        hydrocarbons and the recycle gas is treated so as to lower its        temperature and condense the hydrocarbons. It may be envisioned        to then reheat the stream comprising hydrogen and hydrocarbons        by heat exchange with at least one other gas stream of the        process in order to avoid conveying a cold gas to the PSA unit.        Next, the introduction of these mixed gases at pressure P into        the adsorbent bed allows them to be retreated. During the        adsorption phase, the gas streams are introduced into the bottom        part of the bed in the direction known as the co-current        direction. During this contacting step, the most adsorbable        compounds other than H₂, are adsorbed on the adsorbent and a gas        mainly comprising hydrogen is produced at the pressure P reduced        by around 1 bar of pressure drop. During this step, the hydrogen        produced generally has a purity greater than at least 99 mol %,        preferably greater than at least 99.5 mol %.

So as to obtain an effective purification, the adsorbent of the PSA bedsshould, in particular, allow the adsorption and desorption ofimpurities. The adsorbent bed is generally composed of a mixture ofseveral adsorbents, said mixture comprising, for example, at least twoadsorbents chosen from: activated carbons, silica gels, aluminas ormolecular sieves. Preferably, the silica gels should have a pore volumebetween 0.4 and 0.8 cm³/g and a specific surface area greater than 600m²/g. Preferably, the aluminas have a pore volume greater than 0.2 cm³/gand a specific surface area greater than 220 m²/g. The zeolitespreferably have a pore size below 4.2 Å, an Si/Al molar ratio below 5and contain Na and K. The activated carbons preferably have a specificsurface area greater than 800 m²/g and a micropore size between 8 and 20Å. According to one preferred embodiment, each PSA adsorbent bed iscomposed of at least three layers of adsorbents of different natures.Each PSA adsorbent bed may comprise: in the bottom part, a protectivelayer composed of alumina and/or silica gel surmounted by a layer ofactivated carbon and/or of carbon-based molecular sieve and optionallyin the upper part a molecular sieve layer. The proportions vary as afunction of the nature of the gas mixture to be treated (especially as afunction of its percentages of CH₄ and of C₃₊ hydrocarbons). Forexample, a water-free gas mixture comprising 75 mol % of H₂, 5% of C₃₊and 20% of light (C₁-C₂) hydrocarbons, CO and N₂ may be treated by anadsorption unit whose beds comprise at least 10 vol % of alumina and 15vol % of silica gel in the bottom bed, the remainder being obtained fromactivated carbon.

During the decompression phase of the PSA, the waste gas is produced.This production of the waste gas may be obtained by countercurrentdecompression initiated at a pressure below P. This waste gas comprisesimpurities and has a hydrogen content lower than all the streamsintroduced into the PSA unit. This waste gas may be discharged from theprocess and burnt or reused as a recycle gas in the PSA unit asindicated previously.

The low pressure of the cycle being attained, a purge phase is carriedout to finalize the regeneration of the adsorber. During the purgephase, a gas is introduced countercurrently into the adsorber and apurge gas is produced. The gas introduced countercurrently into theadsorber during the purge phase is a gas stream derived from one of thesteps of the decompression phase. The purge gas is generally used as arecycle gas after recompression.

During the pressure rise phase, the pressure of the adsorber isincreased by countercurrent introduction of a gas stream comprisinghydrogen such as the gas produced during the various steps of thedecompression phase.

According to the main feature of the invention, between step a) ofadjusting the pressure of the gas stream comprising hydrogen andhydrocarbons and step b) of treating this stream via the gas separationunit (U), the temperature of the gas stream comprising hydrogen andhydrocarbons is lowered so as to condense the hydrocarbons, preferablyto less than 0° C., more preferably still to less than −20° C.Preferably, this temperature drop is carried out by means of a cryogenicdevice, such as cryogenic (compression/expansion) loops using liquidammonia or propane.

FIG. 1 illustrates the process according to the invention. The stream 2comprising hydrogen and hydrocarbons is compressed by the compressor 3to have a pressure P equal to that of the stream 1 that mainly compriseshydrogen. The stream 21 comprising hydrogen and hydrocarbons and that iscompressed is then treated by a device 4 that makes it possible to lowerits temperature and to condense the hydrocarbons. The device 4 produces:

-   -   on the one hand, a liquid stream 10 of hydrocarbons (LPG); and    -   on the other hand, a gas stream 22 comprising hydrogen and still        some hydrocarbons having a pressure P and a lowered temperature.

This stream 22 and the stream 1 are treated by the gas separation unit 5so as to produce: a hydrogen-enriched stream 6, a waste gas 7 and apurge gas 8 which is mixed with the stream 2 comprising hydrogen andhydrocarbons before compression of the latter. The process also makes itpossible to treat another stream 9 comprising hydrogen and hydrocarbonsthat is equivalent to the stream 2 but originates from another purge.This stream 9 already has a pressure P or a pressure that is slightlygreater than P; it is therefore directly treated by the device 4 withthe mixture 21 resulting from the compressor.

By implementation of the process as described above, it is possible toreduce the size of the pressure swing adsorption unit and to reduce thecompression costs of the unit (U) while increasing the hydrogen yield.The process additionally makes it possible to produce, at the outlet ofthe temperature-lowering device, a liquefied petroleum gas, which may bea stream of high added value, that can be recovered as a fuel.

The process of the invention also has the advantage of allowing anatural energy integration between the cryogenic device enabling thetemperature to be reduced and the compressor of the pressure swingadsorption unit.

EXAMPLE

Two gas streams comprising hydrogen were treated by means of a pressureswing adsorption unit. These two streams had the followingconcentrations:

TABLE 1 Gas stream mainly Gas stream comprising comprising hydrogenhydrogen and hydrocarbons Flow rate Sm³/h 10 000 5 200 Composition (vol%) H₂ 81.4 57 C₁ 9.3 23.7 C₂ 5 2.2 C₃ 2.6 5.35 C₄ 1.1 8.2 C₅ 0.6 2.9 N₂0 0.6

These streams were treated by a pressure swing adsorption unit accordingto the invention as illustrated in FIG. 1 and by a CPSA pressure swingadsorption unit according to the prior art corresponding to the CPSAaccording to the invention but not comprising means for lowering thetemperature of the gas stream comprising hydrogen and hydrocarbons so asto condense the hydrocarbons. The features of the process according tothe prior art and of the process according to the invention aredescribed in table 2.

TABLE 2 Process according Process according to the prior art to theinvention Flow rate of the gas 3.06 3.06 stream (1) mainly comprisinghydrogen (t/h) Flow rate of the gas 3.48 3.48 stream (2) comprisinghydrogen and hydrocarbons (t/h) Hydrogen yield 82.3% 82.6% Volume ofadsorbent 100 89.5 (relative) Hydrocarbon recovery (t/h) 2.02 mol % C321.2 mol % C4 51.4 mol % C5 27.4 Energy used by the 564 541 compressor(3) (kW) Energy used by the 431 cryogenic device (4) for lowering thetemperature −25° C. (kW)

It is observed that by the implementation of the invention, it ispossible:

-   -   to reduce the volume of the adsorbents of the PSA unit by 10 vol        %;    -   to decrease the power of the compressor by 5%; and    -   to increase the hydrogen yield by 0.3 points.

Furthermore, the stream of liquefied hydrocarbons may be recovered as acommercial product within the plant.

The liquefied petroleum gas resulting from the cryogenic device is aproduct of high added value which compensates for the additional energyused by the cryogenic device of the process according to the inventionrelative to the process of the prior art.

1-7. (canceled)
 8. A process for producing a hydrogen-enriched gasstream and a liquefied petroleum gas from at least one gas stream mainlycomprising hydrogen at pressure P and a gas stream comprising hydrogenand hydrocarbons, in which the following steps are carried out: duringstep a), the gas stream comprising hydrogen and hydrocarbons is adjustedto the pressure P; during step b), the gas stream comprising hydrogenand hydrocarbons and adjusted to the pressure P is treated in a pressureswing gas adsorption unit fed will the gas stream mainly comprisinghydrogen and having a pressure P so as to supply: at a first outlet, thehydrogen-enriched gas stream having a hydrogen concentration greaterthan that of the gas stream mainly comprising hydrogen; and at a secondoutlet, a waste stream characterized in that between step a) and stepb), the temperature of the gas stream comprising hydrogen andhydrocarbons and adjusted to the pressure P is lowered so as to condensethe hydrocarbons and form the liquid petroleum gas.
 9. The process ofclaim 8, wherein the gas stream mainly comprising hydrogen has ahydrogen concentration between 50 and 99 vol %.
 10. The process of claim8, wherein the gas stream comprising hydrogen and hydrocarbons has ahydrogen concentration at least 10% lower relative to the value of thehydrogen concentration of the gas stream mainly comprising hydrogen. 11.The process of claim 8, wherein the gas stream comprising hydrogen andhydrocarbons has a hydrocarbon concentration between 10 and 50 vol %.12. The process of claim 8, wherein the gas separation unit is apressure swing adsorption (PSA) unit combined with an integratedcompressor, in which, for each adsorber of the unit, a pressure swingcycle is carried out comprising a succession of phases which define thephases of adsorption, decompression, purge and pressure rise, such that:during the adsorption phase: during a first step, the gas stream mainlycomprising hydrogen and having a pressure P is brought into contact withthe bed of the adsorber; and during a second step, introduced intocontact with the bed of the adsorber is the mixture of pressure P whosetemperature has been lowered so as to condense the hydrocarbons and thatis composed of: on the one hand, the gas stream comprising hydrogen andhydrocarbons; and on the other hand, the PSA recycle gas,  so as toadsorb the compounds other than hydrogen and to produce, at the top ofthe adsorber bed, the enriched stream having a hydrogen concentrationgreater than that of the gas stream mainly comprising hydrogen; duringthe decompression phase, the PSA waste stream is produced; during thepurge phase, a purge gas is produced; and where the PSA recycle gas iseither the waste stream compressed to the pressure P, or the purge gascompressed to the pressure P.
 13. The process of claim 8, whereinbetween step a) and step b), the temperature of the gas streamcomprising hydrogen and hydrocarbons is lowered to at least 0° C. 14.The of claim 8, wherein between step a) and step b), the temperature ofthe gas stream comprising hydrogen and hydrocarbons is lowered by meansof a cryogenic device.
 15. The process of claim 9, wherein the gasstream comprising hydrogen and hydrocarbons has a hydrogen concentrationat least 10% lower relative to the value of the hydrogen concentrationof the gas stream mainly comprising hydrogen.
 16. The process of claim15, wherein the gas stream comprising hydrogen and hydrocarbons has ahydrocarbon concentration between 10 and 50 vol %.
 17. The process ofclaim 16, wherein the gas separation unit is a pressure swing adsorption(PSA) unit combined with an integrated compressor, in which, for eachadsorber of the unit, a pressure swing cycle is carried out comprising asuccession of phases which define the phases of adsorption,decompression, purge and pressure rise, such that: during the adsorptionphase: during a first step, the gas stream mainly comprising hydrogenand having a pressure P is brought into contact with the bed of theadsorber; and during a second step, introduced into contact with the bedof the adsorber is the mixture of pressure P whose temperature has beenlowered so as to condense the hydrocarbons and that is composed of: onthe one hand, the gas stream comprising hydrogen and hydrocarbons; andon the other hand, the PSA recycle gas,  so as to adsorb the compoundsother than hydrogen and to produce, at the top of the adsorber bed, theenriched stream having a hydrogen concentration greater than that of thegas stream mainly comprising hydrogen; during the decompression phase,the PSA waste stream is produced; during the purge phase, a purge gas isproduced; and where the PSA recycle gas is either the waste streamcompressed to the pressure P, or the purge gas compressed to thepressure P.
 18. The process of claim 17, wherein between step a) andstep b), the temperature of the gas stream comprising hydrogen andhydrocarbons is lowered to at least 0° C.
 19. The of claim 18, whereinbetween step a) and step b), the temperature of the gas streamcomprising hydrogen and hydrocarbons is lowered by means of a cryogenicdevice.